Form W-4
The form you give your employer to set how much federal income tax is withheld from each paycheck.
Form W-4, the Employee's Withholding Certificate, tells payroll your filing status and any adjustments: a second job or working spouse, children and other dependents, other income, extra deductions and any extra tax you want withheld.
If you only fill in your details and sign it, your employer withholds as if you have one job and take the standard deduction. You can hand in a new W-4 whenever your situation changes.
Example
Dana has two jobs. The Step 2 worksheet on her W-4 shows $2,880 of extra tax for the year, so she enters $2,880 ÷ 26 = $110.77 per paycheck in Step 4(c).